1. Technical Field
The present invention relates to fixation assemblies. More particularly, the present invention relates to locking caps for use in screw fixation assemblies for bone fixation systems.
2. Background Art
Several techniques and systems have been developed for correcting and stabilizing the spine and for facilitating fusion at various levels of the spine. Stabilization of the spine for various conditions, including degenerative disk disease, scoliosis, spondylolisthesis, and spinal stenosis, often require attaching implants to the spine and then securing the implants to spinal rods. Such spinal fixation devices can immobilize the vertebrae of the spine and can alter the alignment of the spine over a large number of vertebrae by connecting at least one elongate rod to the sequence of selected vertebrae. These rods can span a large number of vertebrae, such as three or four. The spine anatomy, however, rarely allows for three or more implants to be directly in line. In order to allow for this irregularity, the rod must be contoured to the coronal plane.
Spinal fixation has become a common approach in fusion of vertebrae and treating fractures and the above listed spinal disorders. A common device used for spinal fixation is a bone fixation plate assembly. Typical bone fixation plate assemblies have a relatively flat, rectangular plate with a plurality of apertures therethrough. Additional assemblies include an implantation fixation system that locks a rod to several vertebrae. In these assemblies, as in with other spinal fixation systems, they utilize various fasteners, such as bone screws, to secure the bone fixation plate assembly or the implantation fixation assembly to the desired and targeted vertebrae of the patient. These screws vary in design and shape depending upon their desired location and use thereof.
In particular, polyaxial locking screws are used with these devices. The key to the polyaxial screws used with these systems is having the screw head being securely fastened to the vertebrae and to the assembly thereof. Thus, the polyaxial screws must be used in conjunction with a type of screw head securing device that provides a strong lock to the polyaxial screw. Any movement of the screw can be detrimental towards the healing process of the spine. Further, additional damage can occur if there is movement of the screw once it has been fixed to the vertebrae. Therefore, movement of the screw must be minimized or eliminated.
There are numerous polyaxial screws existing in the market today and known in the prior art. Additionally, numerous devices exist that provide a securing means for locking the polyaxial screw. For example, U.S. Pat. Nos. 5,554,157, 5,549,608, and 5,586,984 all to Errico et al. disclose polyaxial locking screws and coupling element devices for use with a rod fixation apparatus. The '157 patent discloses a coupling element including an interior axial passage having an interior surface that is inwardly curvate at the lower portion thereof such that it comprises a socket for polyaxially retaining a spherical head of a screw. The coupling element further includes a pair of vertically oriented opposing channels extending down from the top of the coupling element, which define therebetween a rod receiving seat. The channel further provides for walls of the upper portion to a pair of upwardly extending members, each including an exterior threading disposed on the uppermost portion thereof for receiving a locking nut. During the implantation of the assembly, the locking nut seals against the top of the rod that in turn seats on top of the screw head. The nut causes the rod to be locked between the nut and the screw and the screw to be locked in the socket.
The '608 patent discloses a modification wherein a locking ring is disposed about the exterior of the lower portion of the coupling element and provides an inward force on an outwardly tapered portion upon downward translation thereof. As a result, the interior chamber crush locks a screw head therein to eliminate the polyaxial nature of the screw element coupling.
The '984 patent discloses a polyaxial orthopedic device including a coupling element having a tapered lower portion having a slotted interior chamber in which a curvate head of a screw is initially polyaxially disposed. The coupling element includes a recess for receiving a rod of the implant apparatus. A locking ring is disposed about the lower portion of the coupling element and provides an inward force on the outwardly tapered portion upon downward translation thereof. The vertical slots are caused to close and crush, thereby locking the screw head within the interior chamber thereof.
U.S. Pat. No. 6,280,442 to Barker et al. discloses a complex locking mechanism having a screw head with a complex head geometry, a crown member, and an outer rigid body. Locking occurs by compressing the crown member against the complex head, which compresses the head against the rigid seat. This compression crushes the machine ridges on the head and secures the screw therein.
Another example of a common locking mechanism is a type of collet that has a spherical seat with a flexible portion that is designed to deflect around the screw. By compressing the flexible portion against a rigid, outer wall, the collet is compressed against the head to cause locking therein. Examples of these collets are found in numerous patents. For example, U.S. Pat. No. 6,053,917 to Sherman et al. discloses a multiaxial bone screw assembly that includes a bone screw having a partially spherical head. Additionally, the assembly includes a receiver member that has a central bore that defines a tapered recess to receive a contracting collet carrying the head of the bone screw. The collet defines a partially spherical recess to receive the head of the bone screw and includes deflectable fingers that substantially surround the screw head. As a set screw is tightened into the receiver member, the set screw compresses the rod against the collet, which presses the collet into the tapered recess of the receiver member, thereby deflecting the fingers of the collet against the bone screw head.
Another patent, U.S. Pat. No. 5,964,760 to Richelsoph, discloses a spinal implant fixation assembly that includes a bone fixation member. A rod receiving seat is operatively connected to the bone fixation element for seating a portion of a rod therein. A locking mechanism, in the form of a nut and locking ring, engages the rod receiving seat for forcing an inner wall of the rod receiving seat to contour around and engage the rod seated therein and for locking and fixing the rod relative to the inner housing. The assembly further includes a screw head receiving insert for obtaining a head of a screw therein. The insert is moveable within the assembly between a locked position entrapping the screw head and an unlocked position wherein the screw head enters or escapes.
Other polyaxial screw patents that utilize a similar collet are disclosed in U.S. Pat. No. 6,010,503 to Richelsoph, U.S. Pat. No. 5,910,142 to Tatar (disclosing the use of a spherical collet that is compressed between the screw head and the rod), and U.S. Pat. No. 5,891,145 to Morrison et al. (disclosing the use of a very complex double wedge locking mechanism).
More specifically, the '142 patent to Tatar discloses a polyaxial pedicle screw device for use with a rod implant apparatus, which utilizes a rod mounted ferrule. The device further includes a screw having a curvate head and a rod receiving body. The body has a rod receiving channel and an axial bore into which the head of the screw is inserted. The rod mounted ferrule seats into a small curvate recess in the upper portion of the screw head such that the rod may enter the body at a variety of angles while maintaining secure seating against the head of the screw. The insertion of a top, set-screw compresses down on the ferrule, locking the rod in position and onto the screw head. Further, the body is locked in position to completely secure the assembly.
The '145 patent to Morrison et al. discloses a spinal fixation assembly that includes a bone engaging fastener and an elongated member such as a spinal rod. The fixation assembly is a multiaxial assembly that permits fixation of the bone engaging fastener to the spinal rod at any of the continuous ranges of angles relative to the rod in three dimensional space. The fixation assembly includes a receiver member having a bore therethrough, the walls of which are tapered near the bottom, and a channel communicating with the bore and having an upper opening at the top of the receiver member for insertion of a spinal rod. An outer wedge member and an inner wedge member are also included. Both members have the general shape of a washer and a bore therethrough. In each wedge member, the respective bore is not parallel to the central axis of the respective wedge member. Additionally, the outside surfaces of the wedge members may be tapered and the respective bores may be tapered so as to self-lock when seated and tightened. The bone engaging fastener fits within the bore of the inner wedge member, which in turn fits within the bore of the outer wedge member, which in turn fits within the tapered sides of the receiver member. When the desired position of the bone engaging fastener in three dimensional space is attained, the components are seated to achieve a tight friction fit.
U.S. Pat. No. 6,063,089 to Errico et al. discloses a polyaxial orthopedic device for use with a rod implant apparatus that includes a screw having a head, a tubular body having holes on top, side, and bottom thereof, and a rod coupling element. The head of the screw is disposed in the body with the shaft of the screw extending out of the bottom hole, such that the body and the screw may initially rotate relative to one another. The rod coupling element has a ball shaped end that sits in the body with the remainder of the rod coupling element extending out of the side hole of the body, such that the rod coupling element and the body are initially polyaxially coupled relative to one another. The ball end of the rod coupling element is disposed on top of the head of the screw. A set screw is provided on top of the body, the tightening of which causes the ball, head, and body to be crushed locked together, thereby preventing further relative motion.
In all of the existing prior art, particularly those described herein, the polyaxial screws utilize a complex locking mechanism and additional locking parts to prevent the movement of the polyaxial screw. Typically, the more complex the locking mechanism, larger components are needed and manufacturing costs are expensive. Locking and achieving strong hold values become more difficult with more parts. Further, the sizes of the various fixation plates and fixation assemblies are critical to these types of surgeries. Bulky components can cause soft tissue irritation, as well as compromise the facet joints at the end of a fusion. Minimizing the size of the implants used is critical in spinal surgery. Soft tissue irritation resulting from extensions of implants is a common occurrence. Many times, it is caused by the implant being thick relative to its environment. For example, implants can be too thick to be sufficiently covered within the muscle tissue. Hence, a reduction in the overall thickness of the implant is a critical advantage.
Accordingly, there is a need for a screw head securing mechanism or device that provides a strong, effective, and secure lock of the screw head in its desired position. Additionally, there is a need for a screw head securing mechanism or device that is minimal in size and has a reduced amount of components to provide for a simpler, more effective, and less cumbersome device for fixing screws.